Generally the 'safe withdrawal rate' for investments is 4% per year. At this rate, the principal will grow a bit above normal inflation, and you'll have basically a guaranteed inflation-adjusted salary.

Someone worth 10M, with maybe 8M of that in liquid investments (i.e. 2M in real estate) could clear about $320K every year. It's good money, but like you said it isn't unfathomable wealth. It is 'fuck you' money, though, where basically anyone can live comfortably without having to take work they don't want to do.

The key benefit is the money is free - it doesn't need to be saved for retirement because the nest egg is already secured, you already own the house and don't have mortgage, any extra money from an external salary just increases the principal, etc.